Picture-transmitting system



Aug. 14, 1928.

H. NYQUIST PICTURE TRANSMITTING SYSTEM Filed Jan. 23, 1925 Y R3) E O & LN m m WW w m A 4 Y B 3 Z w 2 wm u .R M m 0 TM 5 m m W 0 Z Patented Aug.'14, 1928.

UNITED STATES PATENT OFFICE.

HARRY NYQUIST, 01 MILLBURN, NEW JERSEY, ASSIGNOR T AMERICAN TELEPHONEAND TELEGRAPH COMPANY, A CORPORATION OF- NEW YORK.

PICTURE-TRANSMITTING SYSTEM.

Application filed January 23, 1925. Serial No. 4,353.

This invention relates to maintaining a desired level of operatingcurrent at the receiving end of a signaling line, the level of thecurrent over which varies. More particularly, the inventionis concernedwith level control in systems in which line current is employed toproduce at a receiving point an image of an object located at thetransmitting point, as for example, picture transmission systems.

In accordance with the preferred embodiment of the invention, the levelof operating current at the receiver is controlled by line current orimpulses which are used also for synchronizing purposes. Thus. in theform of the invention hereinafter described the current which isemployed to synchronize the two drums of a picture transmission systemis in part diverted at the receiver and the diverted portionautomatically adjusts the level of the picture current which controlsthe light valve used in producing the image.

Referring to the drawings, Figure 1 is a diagram of apparatus at thetransmitting end of the system and Fig. 2 is a diagram of apparatus atthe receiving end.

Light from the source 11 is focused on the small opening in the screen13 adjacent to the surface of the drum 14'around which the picture iswrapped in the form of a semi-transparent film. The drum 14 is mountedon an axial screw-threaded shaft 15 which is rotated by a motor 16 andcaused to advance slowly along its axis with each rotation. Thus thespot of light on the film underlying the opening in the screen 13describes a helical course on the film.

, Within the drum 14 is a photo-electric cell 20 that receives/the lightpassing through the opening of the screen 13 and the underlying film onthe drum 14. According to the well known principle of the photo-electriccell, the current in its circuit varies in eorrespondence with theamount of light falling on it and this quantity of light depends on thedegree of shade in the spot on the film adjacent to the opening in thescreen 13. The varying current from the photo-electric cell 20 goes tothe amplifier 21, and its output goes to the modulator 22, thusmodulating the alternating current from the generator 23. The modulatedoutput from modulator 22 goes through the high-pass filter 24 to theline 19. I

The motor 16 that rotates the drum 14 also drives a low frequencygenerator 17, whose output is interrupted regularly and then goesthrough the low-pass filter 18 to the line 19. For example, the outputof the generator G may be an alternating current of 400 cycles persecond in interrupted wave trains each 1/120 second in duration, and 60of these per second.

Thus the line 19 carries the low frequency synchronizing current wavetrains from the generator 17 and the higher frequency current from thegenerator 23, modulated according to the degree of shade in thesuccessive elements of the picture to be transmitted.

These currents coming in over the line 19 are passed through theartificial line 25 by which their intensity can be adjusted manually.The low frequency synchronizing currents go through the low-pass filter27 and the higher frequency picture currents go through the high-passfilter 26. These picture current-s then pass through the transformer 28to the push-pull detector 29 Whoseoutput is applied through thetransformer 30 and amplifier 31 to the light valve 32.

Light from the source 33 is focused by the lens 3 1 on the light valve32, and in accordance with the degree of the opening of light valve 32the light falls on the underlying spot of a sensitive film wrappedaround the drum The drum 35 is rotated by the motor 37 and traversedalong its axis by the engagement of the screw threads on the shaft 36with a suitable support. The motor 37 is driven synchronously by currentreceived through the low-pass filter 27, and thus the result is securedthat the drum 35 at the receiving station is rotated and traversedsynchronously with the drum 14 at the transmitting station.

A branch circuit from the output of the low-pass filter 27 goes throughthe narrow band-pass filter 38, and the output of this last mentionedfilter goes through the am plifier 39 to the detector 40. This detector40 is adjusted with a high negative voltage on its grid.

Now suppose that for some reason the transmission efficiency of the lineis increased so that an increased intensity of current goes through boththe filters 26 and 27. The effect of the increased current through thefilter 27 is to increase the output current for the detector 40. Thisincreases the potential drop through the resistance 44 and decreases thecurrent in the circuit from ground 42 through battery 43, resistance 44,inductance 46, primaries of transformer 30 and plates to filaments inaudions 29 and thus to ground at 50. Whereas the same cause thatincreases the current through the filter 27 would tend to increase thecurrent through the filter 26 and hence increase the output from thedetector 29, the decreased current in the output circuit of thisdetector that has just been traced will counteract the tendency toincrease, so that the output from the detector 29 will remain constantso far as variations in transmission equivalent of the line and otherparts of the system are concerned.

The inductance 46 and the shunt condenser 47 are provided to supplementthe filter 38 in smoothing out rapid variations in the current.

The output circuit of the detector 40 comprises an indicating meter 41so that suitable adjustments can be made manually at 25 for large slowvariations in line equivalent or when different lines are connected withthe apparatus. The variations for which automatic adjustment is made, asherein disclosed, are often of somewhat transient character and not ofvery wide amplitude.

Instead of applying the current from the battery 43 to the outputcircuit of the detector 29, its electromotive force can be applied tothe input side. This will be accomplished by throwing the switches 45and 49 from the positions shown in Fig. 2. It will readily be understoodhow the decrease of current through the resistance 51 that goes withincreased current received through filter 27 from the line will operateto decrease the output from the detector 29.

I claim:

1. In a signaling system employing current of two components, one fortransmitting message signals, the other for synchronizing, the method ofkeeping the signal current at proper energy level, which consists inautomatically adjusting it by the variation of the synchronizingcomponent.

2. In a signaling system employing current of two components, one fortransmitting message signals, the other for synchronizing, the method ofkeeping the received signal current at proper energy level, whichconsists in filtering the said components into separate paths and thenadjusting the degree of amplification of the signal current inverselyaccording to the intensity of the synchronizing current.

3. In a signal system, means to transmit a current of two components,one for message signals, the other for synchronizing, an amphfier forthe signal current, a detector for the synchronizing current and meanscontrolled by said detector to vary the amplification of the signalcurrent.

4. In a picture transmittin s stem, a light valve at the receiving en ofthe system, means to apply the received picture shade current to operatesaid light valve, and means to vary the intensity of thecurrentcontrolling the light valve inversely according to the energylevel of the current received from the transmitting station.

5. In an ima e producing system, means to generate anti transmit acurrent of two components, one for image production, the other forsynchronizing, means at the receiving end to separate these componentsinto respective paths, means to test the synchronizmg component forintensity, and means to adjust the degree of amplification of the imageproducing component accordingly.

6. In a picture transmitting system, means to generate and transmitcurrent of two components, one modulated for icture shades, the otherfor synchronizing, filters at the receiving end to separate thesecomponents to respective paths, an amplifier for the picture shadecomponent, a detector for the synchronizing component and means toadjust the amplifier according to the intensity of the output of thesaid detector.

7. In a signaling system for producing images at a receiving point of anobject at a transmitting point, the method of keeping the receivedoperating current at proper energy level. which consists in transmittinga non-signaling current from the transmitter to the receiver andutilizing said current simultaneously for controlling the rate ofscanning at the receiver and for adjusting the apparatus at the receiverto vary the degree of amplification inversely to changes in the energylevel of the received image current.

8. Asignaling system for producing images at a receiving point ofobjects at a transmitting point, which comprises means for transmittingimage current impulses, means for receiving and amplifying said currentimpulses, means at the transmitter for transmitting non-signalingcurrent impulses, means at the receiver for utilizing the receivednon-signaling current to control the rate of scanning, means at thereceiver for detecting changes in energy level of the receivednon-signaling current, and means at the receiver to vary the degree ofamplification of said amplifying means inversely to changes in theenergy level of the received image current.

9. A signaling system for producing images at a receiving point ofobjects at a transmitting point which comprises means A for generatingsignaling current, means at the transmitter for generating non-signalincurrent, means for transmitting both of sai currents from thetransmitting to the receiving point over the same transmission line,

filtering means at'the receiving point for "separatin the signalingcurrent and the non-si current, means for amplifying the sal d signalingcurrent, means for utilizing the'received non-signaling current tocontrol the rate of scanning, means for defcc'ting changes in energy.level of the said received non-signaling current, and means atthereceiver to vary t-he degree of amplification of said amplifying meansinversely to changes in the-energy level of received signaling current.1

10. The method of maintaining uniform transmission stem which comprisescontrolling the intensity of light incident upon a receiving surface inaccordance withchan in the transmission level of a line andindependently of image current being received.

11. The method of maintaining uniform reception in an image transmissionstem which comprises controlling the intenslty of the image producinglight at the receiver in accordance with current level changes due tochanges in a transmission characteristic of the transmitting medium andindependently of image current being received.

12. In an image transmission system, "a sending station, a receivingstation, a transmission channel interconnecting said stations, means forsending image current over said channel, means for sending test currentover said channel, a com nsatmg device at the receiving. station, anmeans responsive to the sending of said test current to cause saidcompensating device to maintain uniform contrast relation between thereceived image and the object whose image is transmitted.

13. In an image transmission system, a sending station, a receivingstation, a transmision channel interconnecting said stations, means forsending over said channel current modulated in accordance with the tonecharacteristics of a picture or object means for receiving said current,a source, of light and a receiving medium therefor at the receivingstation, a device at the receivin station adapted to control the amount0 ing to the intensity of the modulated according to the tonecharacteristics of a picture or other object from said sending stationto. said receiving station, means for sending unmodulated carriercurrent to test said channel, means at the receiving station for varyingthe intensity of the light rays from said source in accordance with themodulated current, and means at the said receiving station for varyingthe energy controlling the intensity of the light from said source inaccordance with said test current.

15. In a signaling means for transmitting message signals an means fortransmittin synchronizing current, the method of eeping the receivedsignal current at progxer energy level, which comprises adjusting t edegreeof amplification of the signal current inversel accordsynchromzingcur rent.

16. A signaling system comprising means for transmitting signalingcurrent, means for transmitting synchronizing current, signal amplifyingmeans, means to test the synchronizing current for intensity and toutilize the changes in intensity to adjust the degree of amphfication ofsaid amplifying means.

17. In an image transmission system, a sending station, a receivingstation, means for transmitting image current, means for transmittingtest current, and means controlled b said test urrent for compensatingfor c anges of level of transmitted current to maintain uniform contrastrelation between the image and the object.

In testimony whereof, I have signed my name to this specification this16th day of January, 1925.

HARRY NY QUIST.

system employin

